What xelios said it's pretty much spot on.
A few months ago my bf was texting with his "friend" (while we were watching Devil wears Prada, movie I can't watch anymore, for now), I catched a glimpse of the conversation(he was besides me and he didn't let the effing blackberry off [i hate those things now

]), I didn't say anything at first and was still watching the movie but paying attention to what he was texting, until after a few minutes where it all turned into a date-sex conversation , he thought the whole time I was watching the movie, not paying attention to his doings and also thought I couldn't see what he texted since he adapted a weird angle every time he was typing, but being the stupid beyatch I am, I was able to see the texts, neck and eye pain later

(sacrifices must be made)
I was really nervous at first until that convo reached a new high , so I stopped the movie and asked him to give me his cellphone, he refused telling me he didn't like my tone. Shit-storm started!
I was really sad and started crying, he was crying too saying it was a friend and it was not what I thought that he loved me and all that stuff. That happened at like 11pm, the discussion raged on , now it moved to "I know I'm an idiot I got carried away but he is just a friend! I will stop using my cellphone if that's what you want", I was just too tired and sad not even not paying attention,but just sitting there crying till like at 5 am I told him to go back to his room, he hesitated a while until he left and came back later with his laptop and cellphone, he left them in my room and when to sleep in his (in short: we lived in different rooms, but same building in this lost town here in Mexico, thanks to work

). That night was Saturday, Sunday was the worst day ever, we were supposed to go with friends to a fair in a near town, I stayed in my room all day
I didn't talk to him for the next few days. Days later, at night, he asked me to go with him to the dinning hall , I agreed not saying a word and it was the start of V2 our relationship. Lights off, candles all over the place and dinner , from that day he always prepares breakfast, goes with me to...everywhere (when in the past sometimes he didn't come along), tells me who called him, who texted him, who will he call, oversleeps with me (before he would wake up shower and then wake me up because we had to "get most of the rest days" even if I carried the zombie face everywhere and sometimes I just wanna sleep, really), playing videogames he doesn't like with me(geek

), exercising with me he(he doesn't like exercisie), in the end he is doing many many things to show that he cares.
And yet sometimes I don't feel it, and (as xelios said) feel that I'm disrespecting myself by staying. I told him that and he goes on to apologize and tell me how he messed up and that he knows I don't trust him 100% anymore specially because sometimes I'm cold to him now, but he insist that he will keep trying to make things better than even before this...incident.
Sometimes I think I'm stretching his patience too much, but then again before we were together I told him how I become an annoying bitch that will make him suffer if he cheats on me(my ex also cheated on me). Slowly he is regaining my trust, but he has to keep trying, it's not something that's easily forgotten.
Right now we live in different cities , and he comes most weekends, and when he cannot come, he calls whenever he is going out with friends or to a party. Adding to my bitchiness, he has to answer whenever I call, or call back asap, and turn on the webcam whenever I want

and I can see whats on his cellphone whenever i want to. Not that I do it regularly, but promises are to be fulfilled.
So yeah, that was part rant part advice. But do no take a decision when you are emotionally weak, specially cause most decisions will lead you to the "I can't live without him"